1. Field of the Invention
The present invention relates to print data processing techniques and in particular to print data processing method and apparatus which convert print data to two-level or binary data for printing with halftone reproducibility.
2. Description of the Related Art
There have been proposed various print data processing techniques for reproducing halftones in, for example, an electrophotographic printing device. One of such techniques employs a halftoning scheme which involves amplitude modulation screening as proposed in, for example, Japanese Patent Application Laid-open Publication No. 5-252386 and another one employs an error diffusion scheme involving frequency modulation screening as proposed in, for example, Japanese Patent Application Laid-open Publication No. 8-298595.
The halftoning scheme sets threshold values dot by dot, and compares input data with the threshold values dot by dot to thereby form halftone cells. Since the halftoning scheme generally carries out development while increasing the number of dots outward from the center, it matches the developing characteristic of the electrophotography technique.
The error diffusion scheme, by way of comparison, acquires the binary error of a specific pixel and distributes the error over other pixels around that specific pixel in accordance with the computed densities by using weights which have been obtained empirically. Theoretically, this error diffusion scheme can freely provide a halftone level.
However, the halftoning scheme requires the increased size of halftone cell in order to increase the number of displayable halftone levels, so that the print result is likely to have graininess or a dispersed-dot appearance. Reducing the sizes of halftone cells to decrease the degree of such graininess however causes the number of displayable halftone levels to be reduced, thereby degrading the print quality.
On the other hand, the error diffusion scheme suffers such a shortcoming that a low-density portion is not easily developed due to dot diffusion while a high-density portion becomes dark due to interference of a photosensitive body and is thus easily developed. The contrast of the print result therefore gets too high. In addition, because of the necessity to compute an error dot by dot, the error diffusion scheme would take a longer processing time than the halftoning scheme.
The conventional technique disclosed in the aforementioned Japanese Patent Application Laid-open Publication No. 8-298595 has not yet overcome the inherent problem of the error diffusion scheme of developing a high-density portion darker while having improved the dot-diffusion originated reduction in the density of a low-density portion. Further, since the error diffusion scheme does not consider the dot gain at the time of development after diffusing an error over input data, it is expected that the number of displayable halftone levels in actual printing becomes smaller than the theoretical value.
The conventional technique disclosed in the aforementioned Japanese Patent Application Laid-open Publication No. 5-252386 has not yet succeeded in shortening the processing time in error diffusion for each halftone cell.
Accordingly, it is an object of the present invention to provide a print data processing method and apparatus which overcomes the aforementioned drawbacks and can ensure high-quality printing with improved halftone reproducibility and less graininess or dispersed-dot appearance, can ensure stable print quality secure and can improve the printing speed.
According to the present invention, a print data processing method for a printing device includes the steps of a) storing error data in halftone level between an actual halftone characteristic of the printing device and a target halftone characteristic; b) dividing input print data into halftone cells of a predetermined pattern to produce halftone-cell data for each of the halftone cells; c) sequentially diffusing the error data of a halftone cell over halftone-cell data of another halftone cell using a predetermined distribution ratio from the halftone cell to the other halftone cell to correct halftone-cell data of each of the halftone cells; and d) producing dither data of each of the halftone cells from corrected halftone-cell data thereof.